Vertical shaft impact crushers are generally known in which centrifugal force is used to hurl large rocks against an impact surface, thereby to obtain smaller crushed rocks. Rock material is typically fed into a rotating impeller which hurls the rock material against a plurality of anvils disposed about the impeller. In the alternative, the rotating impeller throws the rock material against a bed of already crushed rock instead of the anvils. In either event, the rock crusher processes relatively larger rock material into relatively smaller crushed rock.
One important consideration in the design of rock crushers is the extension of the useful life span of the equipment. It will be appreciated that certain of the components come into direct contact with the rock material and, therefore, are subject to wear. Accordingly, the wear components are typically releasably attached to the rock crushing apparatus so that they may be removed and replaced. Other components are intended to be permanent, and therefore must be protected from direct contact with the rock material. The non-wear components are usually more permanently attached to the crusher apparatus.
For example, in a vertical shaft impact crusher of the "open table" type, the rotating impeller comprises a generally flat table having multiple shoe assemblies projecting from a top surface of the table near its periphery. The shoe assemblies typically comprise a support bracket attached to the table and a shoe releasably secured to the bracket. Rock material is dropped near the center of the table and, under centrifugal force, moves toward the periphery of the table where the shoes direct the large rock material toward an impact surface surrounding the table assembly, typically an anvil ring. The table is mounted on a flywheel attached to a rotating shaft. In this example, the shoes and anvil ring contact the rock material and therefore are wear components which should be attached to the crusher apparatus in such a manner that they are easily removed and replaced. The table, flywheel, and shaft are shielded from direct impact and therefore are more permanent, non-wear components.
The table of the above-described rock crusher experiences significant stress during operation. The shoes are mounted near the periphery of the table and therefore create a bending moment in the table. An additional downward bending moment is created when the table rotates due to centrifugal forces acting on the shoes projecting from the top surface of the table. Additionally, the impact forces of the aggregate material against each shoe creates a bending stress in the table about a radial axis extending from the table center to the shoe.
It is often desirable for a crusher to be capable of crushing increasingly larger incoming rock material. In this event, conventional rock crushers often use a larger table to increase the speed at which rock material is thrown at the impact surface, thereby to more effectively break the rock material. The larger table, however, requires a corresponding size increase in many of the other components in the crushing apparatus, and therefore is not suitable for retro-fit installation.
One approach to improving the performance of the crusher while maintaining the overall size of the apparatus is to increase the rotational speed of the table. While the increased rotational speed increases the speed of the rock material striking the impact surface, the stresses in the table increase. Thus, there is an increased chance of table failure caused by the high resulting stresses.
The stress in the table is further affected by the load carried by the table. For example, the shoes may be attached to the table using threaded fasteners rather than pins. The threaded fasteners require mounting brackets which mate with the removable shoes. The mounting brackets for the threaded connection, however, have a significantly higher mass than the pin-type shoe brackets, and therefore the load carried at the periphery of the table is increased.
The risk of table failure is often greatest when a table of a vertical shaft impact crusher is retrofitted to operate at a higher rotational speed or with heavier, fastener-type shoes and brackets. In such a procedure, the table is not typically replaced since it is a non-wear member. Accordingly, the table is subjected to higher stresses than originally intended, and therefore the likelihood of failure is increased.